Maybe so, but their interpretation of their religious book is a major factor. For example, one of the commandments is "Thou shalt not kill", but some people seem to interpret it as "Thou shalt not kill, unless the other person doesn't agree". Similarly, the Bible is all about how you shouldn't sin and all that - some people take this to mean just what it says - that they, themselves, shouldn't sin - while others take this to an extreme and decide to actively, violently "punish" those they believe have sinned.

The Bible says you should not kill, and it also says that those who use it for evil intent or just to hurt others shall be punished. But I'm talking about extremists. Even the Bible has some extreme things in it that go back to the Torah but at least the Bible later speaks against this. Not all religious books with violence speak against it. And terrorists are living out and fulfilling parts saying to kill.